In the interest of editorial simplicity, this specification will focus on the production of "bathroom tissue". However, where the context allows, it should be understood that the technological principles of the present invention apply also to the manufacture of all lightweight absorbent grades of paper, including, for instance, facial tissue and paper toweling.
Paper is generally manufactured by suspending cellulosic fibers of appropriate length in an aqueous medium and then removing most of the water to form a web. The paper derives some of its structural integrity from the mechanical arrangement of the cellulosic fibers in the web, but most by far of the paper's strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this interfiber bonding, while necessary to the utility of the product, results in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to a Yankee drum thermal drying means with an adhesive/release agent combination and then scraping the web off of the Yankee by means of a doctor blade. Creping, by breaking a significant number of interfiber bonds, increases the perceived softness of resulting bathroom tissue product.
Obtaining and maintaining adhesion of tissue and towel products to Yankee dryers is an important factor in determing crepe quality. Inadequate adhesion results in poor or non-existing creping, whereas excessive adhesion may result in poor sheet quality and operational difficulties. Traditionally, creping adhesives alone or in combination with release agents have been applied either to the sheet or to the surface of the dryer in order to provide the approriate adhesion to produce the desired crepe. Various types of creping adhesives have been used to adhere fibrous webs to dryer surfaces such as Yankee dryers. Some examples of prior art creping adhesives rely upon combinations of self-crosslinkable soft polymers having a T.sub.g of less than 10.degree. C. with a non-film forming hard polymer emulsion having a T.sub.g of greater than 50.degree. C. (U.S. Pat. No. 4,886,579). Some others involve thermoset resins (U.S. Pat. Nos. 4,528,316 and 4,501,640). The ability to control the mechanical properties of the polymers, as well as the adhesion and release of the fibrous web from the Yankee dryer, is limited when using these types of creping adhesives.
U.S. Pat. No. 5,246,544 describes an improved creping adhesive that provides the ability to readily control coating mechanical properties and adhesion, and which can be more easily removed from dryer surfaces. The adhesive system described in said patent provides high adhesion of a fibrous web to a dryer surface with low "friction" . Having low friction means that the fibrous web can easily be removed from the dryer surface. This can be accomplished while at the same time reducing or inhibiting corrosion of the dryer surface. This is accomplished because the adhesion properties of the types of polymers taught to be useful according to the patented invention can be systematically changed by varying the amount of crosslinking that may occur when the polymer is dried onto the surface of a Yankee dryer. Because crosslink density influences the mechanical properties (that is, modulus, tensile strength, elongation, brittleness, T.sub.g), this permits the adjustment of adhesion/release of the fibrous substrate onto the surface of the dryer. The nature of of the polymers and the types of crosslinker used according to the patent permits the incorporation of anti-corrosion components in the adhesive formulations.
Doctor blades are commonly used for effecting creping of paper in papermaking machines. Inasmuch as a doctor blade is normally in contacting relation with the surface of a rotating cylinder, the tip of the doctor blade is subject to wear. As wear progresses, the doctor blade's effectiveness tends to diminish. The doctor blades used for creping paper in a tissue papermaking machine precipitate progressively greater loss of machine-direction tensile strength of the paper as doctor blade wear progresses. This is particularly true in installations wherein the impact angle progressively changes as wear occurs. Commonly, in such machines, creping blades are changed (that is, replaced by new or newly sharpened blades) after a product property of particular importance (for instance, machine direction tensile strength) has been reduced to a predetermined minimum acceptable level by doctor blade wear, or after other observed deleterious ramifications of abnormal doctor blade performance. Changing creping blades requires considerable down-time and slows production greatly.
It is an object of the present invention to provide a creping adhesive system that will decrease adhesive-system-related wear on the doctor blade in tissue creping operations. The use of creping adhesive formulations in accordance with the present invention extends effective blade life by decreasing the overall level of blade wear while increasing the length of time that the creping doctor blade retains its uniformity.